This invention deals with a method of improving the performance of a process for the manufacture of alkylhalosilanes. This invention deals mainly with the production of silicon, which is used in the direct process for the production of alkylhalosilanes. More particularly, this invention deals with a method of controlling the level of phosphorous promoters in silicon used in the direct process, in order that the direct process in which the treated silicon is used is enhanced in terms of reactivity and selectivity of the direct process reactions.
The benefits to be derived by the use of this invention are increased alkylhalosilane yields, selectivity of certain alkylhalosilanes over other, less preferred. alkylhalosilanes and, overall high utilization of raw materials used in the direct process reaction mixture.
The direct process for producing alkylhalosilanes is well-known and has been refined and modified in many ways since Rochow first set forth the manner in which one could obtain alkylhalosilanes by contacting alkylhalides with silicon at elevated temperatures. In U.S. Pat. No. 4,602,101, issued July 22, 1986, in the name of Roland L. Halm, Oliver K. Wilding, Jr. and Regie H. Zapp, there is disclosed the use of certain phosphorous compounds in the direct process in the presence of silicon, copper and tin, to enhance the reactivity and selectivity of the reaction to produce alkylchlorosilanes. Such phosphorous compounds are selected from elemental phosphorous, metal phosphides and phosphorous compounds capable of forming metal phosphides in the reaction mass of the direct process.
Early investigators dealt with the problems of enhancing the reactivity and selectivity of the direct process by approaching the problems from the standpoint of the physical forms of the raw materials; the treatment of the surfaces of the raw materials or the inclusion of components other than silicon and copper in the reactor feed. Thus. Nitzsche, in U.S. Pat. No. 2,666,776 issued Jan. 16, 1954, teaches that alloys of silicon and copper which also contain metals from the 5th to the 8th groups of the periodic table such as, for example, cobalt, nickel, iron or phosphorous increase the efficiency of the process if an activator, for example, a copper salt is also used.
Zoch, in U.S. Pat. No. 3,446,829, issued May 27, 1969, teaches a contact mass for the direct process containing silicon, a copper or silver catalyst and a cadmium promoter, This combination can be used as a powder mix or an alloy.
Rossmy, in German ALS No. 1,165,026 teaches doping of silicon by sintering powdered silicon or ferrosilicon with powdered copper alloys containing certain additives. Such additives have been described as antimony, indium, thallium, gallium, phosphorous, arsenic and bismuth. Also, in Soviet Inventions Illustrated, General Organic Section, February 1966, page 2, there is essentially described the Rossmy teaching wherein antimony and phosphorous are used in combination as an alloy, with silicon and copper,
And finally, in an article entitled "Influence of Additions of Some Elements to Silicon-Copper Alloys on Their Activity in the Reaction with Methyl Chloride," Lobusevich, N. P. et al., translated from Zhurnal Obshchei Khimii, Vol. 34, No. 8, pp 2706-2708, August, 1964, silicon-copper alloys are described in which certain additives are used in conjunction therewith to enhance the direct process. The article shows phosphorous to be a catalytic poison at concentrations of 50 to 80 ppm based on the alloy. Further, it is noted in the summary that phosphorous when added to alloys in addition to promoters, considerably improves the catalytic property of the silicon-copper alloys. It fails, however, to suggest which promoters will or will not improve this property.
Thus, collectively, the prior art teaches that combinations of silicon-copper alloys and certain other materials can be used to affect the reactivity or selectivity of the direct process. These combinations can take the form of alloys or mixed powders, or the like, and can be used directly in the process. All of the prior art teaches alloys i.e. the melting together of certain components, but the prior art does not teach the production of silicon for the direct process wherein the level of phosphorous in the silicon is controlled such that known amounts of phosphorous are introduced to the direct process reactor with the result of enhanced reactivity and selectivity.
In the instant invention, it was found that the phosphorous, which has the beneficial effect on the direct process, can be introduced into the silicon at the levels required for the direct process, by controlling the amount of phosphorous that enters the silicon during the smelting of the silicon, It has been discovered that certain quartz deposits contain phosphorous which survives the smelting process. In addition, certain phosphorous compounds can be added to the raw materials that go into the silicon smelting in order to obtain phosphorous in the final silicon product. Further, it has been discovered that phosphorous is also a contaminant in the raw materials that are used during the smelting of silicon, such as, for example, the carbonaceous reductants used in the smelting process.
Thus, the instant invention deals with a method of improving the performance of a process for the manufacture of alkylhalosilanes, said process comprising, contacting an alkylhalide with silicon, at a temperature of 250.degree. to 350.degree. C., in the presence of tin or tin compounds, and copper or copper compounds, wherein there is at least also present, 25 to 2500 parts per million, based on the silicon in the reaction mass, of a phosphorous promoter, which method comprises controlling the level of the phosphorous promoter in the silicon by incorporating and controlling the level of phosphorous in the mass of the silicon, by controlling the amount of phosphorous that is introduced into the silicon during the manufacture of the silicon.
The silicon manufactured in this manner has been found to contribute to enhanced reactivity and selectivity in the direct process for the production of alkylhalosilanes.